1. Field of the Invention
The present invention relates to a memory. More particularly, the present invention relates to a nonvolatile analog memory.
2. Description of Related Art
With an advancement of electronic technologies, electronic devices have become indispensable in our daily lives. Owing to increasing demands on information, almost all of the electronic devices currently on the market are equipped with memories for data storage. In the past, various digital memories have been widely proposed. The digital memories include dynamic random access memories (DRAMs) and read only memories (ROMs).
However, one of the common issues encountered by the aforesaid digital memories lies in that one memory cell is required for correspondingly storing each bit of data. Nowadays, as memory capacities required in the electronic products are increasing, more and more memory cells are required. Accordingly, physical dimensions of the memories are also increased. The increasing dimensions of the memories lead to difficulties not only in designing the memories but also in controlling manufacturing yield of memory chips. Moreover, it is more challenging to design chips in which embedded memories are needed.
In view of the above, some nonvolatile analog memories in which stored data is represented by an analog signal are further developed. It is well known that when the data to be stored is in a form of the analog signal, only one memory cell is required for storing a plurality of bits of data, whereas a plurality of memory cells are needed to store the plurality of bits of data in the conventional digital memory. As such, the required number of the memory cells can be significantly reduced, and the area occupied by circuits can also be decreased.
The amount of the bits of data which can be stored in a single memory cell of the analog memory is determined upon the accuracy of the analog signals stored therein. Nonetheless, it is rather difficult to store an accurate analog signal in the analog memory. First, it should be determined whether the written voltage or the written current value is accurately identical to the voltage or the current value representing the data intended to be written. Moreover, it should be further assured an excessive drift of the written voltage/the written current value does not occur when reading the data stored in the memory. Otherwise, the stored data is likely to be altered.